Electrical apparatus



Feb. 11, 1936. J, w, DAwgoN 2,030,099

ELECTRICAL APPARATUS Filed Jan. 6, 1933 S Sheets-Sheet 1 Ff .l.

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Feb. 11, 19.36; I J. w. DAWSON 2,030,099

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' ELECTRICAL APPARATUS Filed Jan. 6 1933 3 Sheets-Sheet 3 Power Source Fbwer Source WITNESSES: INVENTOR 4] 1 Jofm VI. Dawson Patented 1- .5. 11, 19 36 UNITED STATE/S smo'rmcsamm'rus John W. Dawson, Wiikinsburg, Pm, asslgnor to Westinghouse Electric & Manuf acturing Oompany,-East Pittsburgh. Pa., a corporation of- Pennsylvania Application January 0.103s. Serlal No. 050,419

13 Claims. (01. 171-110) My invention relates to transformer tap-changing systems and has particularrelati'on to systems employing vapor-arc or electronic conduction devices for changing the active number oi turiisln a transformer winding without interrupting. the load current thereof. a

In changing the circuit'connections to transformer taps under load, as is requiredto meet the conditions in the power circuits connected to the transformer, switching means capable of. operating in a current-carrying circuit are required. In all such tap-changing systems utilized in the past, with which I amfamiliar, these switching means have been contactor type switches or circuit breakers employing well-known mechanically separable contact members. My invention contemplates replacing the conventional contactor type of switch, in transformer tap-changing sys-,

tems adapted to operate under load with control-' lable vapor-arc or other suitable electronic conduction devices.

In a copending application Serial No. 626,866,

by Joseph Slepian and Leon R.. Ludwig, filed July 30, 1932, and assigned to the Westinghouse Electric and Manufacturing Company there is disclosed a form of vapor-arc electronic conduction device which operates with a'stationary poorly conducting make-alive element, which device pos-. sesses controllable and'other characteristics that make it particularly suitable for use in strans former tap-changing systems. Hence, in disclosing my invention, the electronicdevices illustrated and described are of the above-referred to type. P

The tap-changing systems with which they are shown as forming a part are ofthe single-winding type. It will be apparent, however, as the description proceeds, that my invention may also be practiced with the use of other types of electronic conduction devices, having proper characteristics,

' and that the basic principles thereof are in no way restricted to single-winding tap-changing systems V In the above-identified copend ing application the type of vapor-arc device disclosed employs a made-alive .element which must be excited to initiate the current-conductin pe od of each half cycle. Consequently in the absence of the excitation of this element the device will function as a complete insulator. wave conducting devices .or. by using a doubleended construction in the two ends of which conduction is eifected for alternate half-cycles, conduction. during both half cycles of the alternating current cycle may be'eifected, which conduction' is controllable by the make alive element circuit 1 practical.

By combiningtwo half which may be of low capacity. Hence, by connecting suitable switching means in this circuit, relatively large amounts of power-may be interrupted. or permitted to flow through the device,

and the functions of a conventional type of con- 5 tactor switch are thus effectively simulated. Since the-voltage drop through a vapor-arc device of the type under consideration is relatively low and practically independent of current magnitude, the

powerlosses set up. therein are small, especially 10 when the controlled power circuitsv operate at the relatively high voltages at present encountered in transformer tap-changing applications;

when properly controlled,yapor-arc devices of the type under consideration are capable of allow- '15 ing theactive power circuitconnection to be, shifted from one winding tap'to another during the zerocurrent period in the alternating-current cycle, thereby simplifyingthe operation of tap changing apparatus adapted tofoperate under load. In the 20' past, the use of step bridging auto-transformer has been required in the single-winding tapchanging apparatus and, the double-winding apparatus employs-duplicate windingsin the main changing systems possessing heretoforennknown 'fiexibility characteristics. which, together with the simple control arrangements for the devices, make a tap-changing One objector my invention to provide a tapchang'ing system capable of servicein loadedcircults, in which all switching in the power circuits is-eflected by the use ofcontrollable vapor-arc or to electronic conduction devices. Another object of my invention is to provide a tap-changing system in which a shift from one tap to another may be completely eii'ected when 1 the load current passes through zero.-

1 A further obiect of my invention is to provide a transformer tap-changing system capable of opcrating under loadin which large blocks of power may be controlled by the actuation of low-capacity switching means.

vide a tap-changing system in'which the flexibility characteristics are substantially broadened.

My invention, together with additional objects t and advantages, will bestbe understood from the 50 iollowing description of specific embodiments thereof, when taken in coniunction with the ac-- in which Figure 1 is a diagram of "apparatus and circuits illustrating controllable vapor-arc conduction de- 56 system utilizing themf highly r 5 An additional obiect'of my inventionis to provicesgarranged in a tap-changing system for a single-phase power transformer in accordance with my invention; l

Fig. 2 is a diagram of voltage and current waves illustrating the characteristics of one of the fullwave conductive vapor-arc devices shown in end of the device, which element .two illustrated in Figs. 1 and 4;

Fig. 1;

Fig. 3 is a table setting forth the sequence of control operations required to eflct changesin ggansformer tap connections in the system of Fig. 4 is a simplified representation of one of the vapor-arc devices of Fig. 1 illustrating certain additions to the make-alive element control cl1'- cuits therefor;

Fig. 5 is a similar representation of one of the double-ended vapor-arc devices of Fig. l modified to have but a single make-alive element in each may replace the Fig.6is a. =1 ticshowingoftwohalfwave conducting vapor-arc devices interconnected in a manner to jointly eii'ect full wave conduction;

7 is a diagrammatic view of a tap-changing. system which utilizes two full-wave conducting vapor-arc devices together with a plurality of transformer tap-selector switches of the conventional type; 1

Fig. 8 is a ditic showing of apparatus and circuits illustrating an an'angement of control switcheswhereby a plurality of vapor-arc devicesutilized in a tap-changing system of the type shown in Fig. 1 may be excited frmn but single set of exciting transformers, and I Fig. 91s a diagrammatic view of apparatusand circuits showing a single-winding tap-changing system utilizing a tap-bridging auto-transformer,

s in which vapor-arc devices have been substituted for the conventional contactor type of tap selectin'g switches.

Referring to the drawings, and particularly to 18. 1 thereof, a power transformer I0 is shown as connected between alternating-current power circuit conductors l I and load circuit conductors l2. The secondary winding I! of the transformer is provided with tap connections I and 2. to which the load circuit l2 may be selectively connected through the controllable vapor-arc devices A and B respectively.

As shown, each of the devices A and B, which is capable of full-wave or symmetrical conduction, is of the double-ended type comprising an inverted U-shaped tube ll of insulating material, such as glass, having pools I6 and ll of mercury, or other vaporizable reconstructing material, disposed in the two ends thereof. In contact with each of the pools is disposed a pair of make-alive elements is and II of carborundum crystal or other poorly conducting material, more completely dea separate controlling influeme. It will be recog nized that each of the symmetrically conducting devices in Fig. ithus embodies the multiple makealive element principle extended to vapor are devices of the double-ended or symmetrically conducting type.

Ina vapor-arc device of the above-described typ current conduction between the anode and cathode elements cantake place only when the make-alive element associated with the 'cath,- ode is made active to liberateions from the cathode material. Consequently, until an energizing current is applied to the make-alive elements,

each of the devices A and 13 shown in Fig. 1 will act as an insulator during .both half cycles of alternating-current voltage, and even though the associated pool operating as an anode without a cathode spot thereon. Thus the desired symmetrical conduction -feature may-be obtained by exciting a make-alive element in one end of' the device during alternate half cycles of alternating-current voltage and a make-alive element in the other end of the device during the remaining half cycles. a v I To supply exciting currents to the inner makealive elements I! of devices A and B in the system of Fig.1, exciting transformers 22 and 24- are provided, while transformers 26 and 28 are similarly associated with the 'outer make-alive elements IQ of the two devices named. As shown, each exciting transformer comprises a primary winding 30 and twosecondary windings 31, each of which secondary windings is disposed to impress between a make-alive element and the associated mercury pool of the vapor-arc device an exciting voltage. cluded in each make-alive element circufi: for the purpose of limiting the current supplied thereto.

The primary windings of transformers 26 and '20 are disposed to be energized, through circuits which include switches a and 0, respectively, by

a voltage derived from the power-source circuit, |.I either through an auxiliary secondary winding of power transformer ill or through the medium of a direct connection ,withthe circuit conductors as illustrated. The primary winding of exciting transformer 22 is arranged to be energized through a circuit, which includes a control A suitable resistor 33 is in-.

switch 6, bythe voltage appearing between the major element pools l5 and I 6 of vapor-arc device B, while transformer 24 may similarly derive its energization, through a circuit including a control switch b, .from the voltage appearing between the major element pools of device A.

Upon a proper excitation of either pair of make-alive elements I! or I9 of vapor are device A or B, the deviceis caused to conduct between the two mercury pools thereof a p0wer-circuit current which may be designated by I in Fig. 2,

in which figure 'wave E represents'the trans former voltage applied to the tap circuit which includes the device. I, there is set up in the device a small arc drop of the general character indicated by wave 25 of Fig. ii. This are drop will be observed to be a function of the current, it being in phase therewith ratherthan with the voltage E, which is As a result of this current.

shown as leading the current by an angle alpha 9,080,600 is necessarily present, this arc-drop curve-has the e peakaillustrated at 20.

The manner of operating the tap-changing system of Fig. 1 will be more clearly understood by reference to the control-sequence table of l 'lir. 3. As there set forth, when it is desired to connect'load circuit l2 to tap l of the power transformer ll, control switch a isclosed. This connects exciting transformer to power source conductors II and effects the excitation of outer make-alive elements I! of vapor arc device A.

As a result of this excitation, device it becomes a full-wave conductor, thereby causing the load circuitto be connected to transformer-tap I.

To shift the load circuit connection from tap ltotap2-.switchbisnextclosedtoconnect exciting transformer 21 acrossthe maior ele- I ments of deviceA. 'l'hearc drop through the device in its conductive state, being of but a relatively small magnitude, is'insumcient to effect the active excitation of the inner make-alive elements ll of vapor arc device B so that device B still retains its insulating characteristics.

Switch a'is next opened and the excitation of make-alive elements n of device A is thereby removed. The device accordingly ceases to conduct current when the power current wave next passes through zero with the result that during the nextsucceeding half cycle the voltageyacross the device no longer follows wave II of Fig. 2

but starts to follow wave E. At an early point in the half cycle, this. potential attains a magnitude which is sufiicient to activelyenergiae makealiveelements ll ofdevice B, and thereby render it conductive. Such excitation being repeated at the start of each succeeding half cycle, the device B will by this means be rendered conduc tive, thereby establishing the connection of loadcircuit I! with transformer tap remake the conduction of device B independent, of the potential across device A, switch 0 is now closed causing exciting transformer 28 to be energized from power-source conductors l|. Outer make-alive elements I! of device B are now made active to maintain the device conductive so that control switch b may now be opened to remove excitation from the inner --ma,ke-alive elements ll.

'Iheload circuit l2" now being supplied from transformer winding tap 2 through device B, to

shift the connection from tap 2 back to tap I, switch d is first-closed to connect exciting transformer 22 across the major elements of device B. Switch c is then opened to remove the excitation from the make-alive elements ll of device B which allows the potential across this device to build up sufilciently to excite make-alive'elements ll of device A. This excitation renders device A conductive. Switch 0 is then closed to efi'ect the excitation of the outer make-alive'elements II by the power-source voltage, whereupon switch deenergize exciting transformer 22.. a v 4 "'Iodisconnecttheloadfromthetap i itis onlynecessaryto'openswitchmwhiletosimilarly disconnect it'from tap'lan opening of svvitchcwillsuiilce.

It will be understood that means other than thosespecificallyillustratwasbeingdependent upon device potential, for efiecting the excitation ofone vapor-arc deviceinresponsetothedi'scontinuance of conductivity of-another device may also be utilised in the tap-changing system .my invention. The important typified lay-the illustrated means. is

which, insofar as I am able to learn,-is capable of attainment only through the utilization of controllable electronic or vapor-arc conduction devices in place of tap switches of the conventional or mechanical type. I i Ashasbeenexplained.thearcdropacros sthe vapor-arc device when conductive is ordinarily insufilcient to effect. in a system of the type shown in Fig. 1, excitation of a second vapor-arc device when the excitation transformer of the second device is energized by the potential drop across the first device. In certain instances, however, the peak 38 (Fig. 2) of the arcdrop wavenf-the first device may reach such a mage nitude as to effect active excitation of the second device unless precautionary measures are resorted to. JIhe showing of. Fig. 4 illustrates one such measure which may be utilized to overcome this difiiculty. .In the supply circuits of the make-alive elements II and II are disposed inductive impedances ll in combination with capacitors 02 which act to delay the building up of the make-alive current at thestartof each half-cycle. This delay serves to prevent the voltage peaks in the arc drop through one device from effecting active excitation of an associated device until after the first device is rendered completely non-conductive.

The symmetrically conducting vapor-arcdevices thus far described have beenprovided with two make-alive elements in contact with each of the mercury pools. In certain'instances it may.

be found that a single make-aliveelement may instead be substituted if the excitation circuits associated therewith are properly modified. In

- Fig. 5, a single: make-alive element in each the two separateexciting potentials'from inter- 1 fering with each other. with such properly proportioned circuits the single make-alive element ll maybe made to perform the same functions as do the two make-alive elements II and it in the previously described vapor are devices. The

sensitivity, however, of the excitation system of Fig. 5 is not as high as'thatof the double makealive element arrangement which, in general,

will be found preferable. It will be appa nt.

however, that vapor-arc device A of Fig. 5 and the special excitation circuits associated therewith may-be utilized to replace device A or device B in the system desired.

In Fig.6 I have illustrated two half-wave or asymmetrical conducting vapor-arc devicesll and 46 inter-connected in a manner to effect full-wave Bach of the devices is provided with a cathodepool ll. of mercury or ofI'lg .-l,shouldsuchbe other vaporiaabie reconstructing material and an anode element 4.0 spaced in addition .to a pairofmahe-alive elements lune is, en

of which are enclosed in a suitably sealed container 41. It will be observed that thea'node element 49 of the device 46 is connected through conductor 50 with the cathode element 48 of device 45, while anode 4;! of device 45 is similarly connected with cathode 48 of device 46, con nections with the main power circuit to be controlled being made by means of conductors I3 and 54. Excitation of the make-alive elements of Fig.1 maybe extended to include larger numbers of taps merely by adding more vapor-arc devices and associated excitation means. However, when it is desired to provide for more than two taps with but two vapor-arc devices, the system shown in Fig. 'l in which a plurality of" tap selector switches 51 to 8| inclusive, are incorporated between the devices A and B and the taps l to 5', inclusive, of the secondary-winding of power transformer Ill may be utilized. In-

asmuch as the excitation equipment for the make-alive elements l8 and I! of vapor are devices A and B may be identical with that "which has already been described in connection. with V Fig. 1, no showing of this equipment has been made in Fig. 7

In operation of the systemof Fig. '7, some, ,tion of the load circuit I! with tap I is effected by first closing tap-selector switch 51 and then exciting, from the power-source voltage, vapors arc device A to render it conductiver Transfer source influenced circuits.

gization.

of the load circuit connection from tap I to tap 2 iseflected by closing tap-selector switch 54,, setting up excitation circuits for vapor-arc device B responsive to the potential across device .A, removing the excitation from device A which renders device B conductive and then transferring the excitation of device B to power- Transfer of the load-circuit connection from tap I to tap I may similarly be effected by.openlng switch 51 closing switch 5!, setting up exci-" tation circuits for'devlce A which areresponslve to the potential across device B, removing the excitation from device B torender device A conductive' and then transferring the excitation clrcults .of device A to power-source voltage ener- It will'be apparent that further shifts of power transformer winding may be effected by aslmilar manipulation of the tap-selector switches 'assoelated therewith and alternately rendering the vapor-arc devices A and B conductive in proper order. Shifting the tap connections lnithe reverse or voltage decreasing direction will be seen to be eflected by a modified manipulation ofthe tap selector switches and a proper control of vapor-arc device excitation.

In instances in which the arc drop through the state power supply to theload circuit, device shunting switches may be provided. In the system of Fig. '7, such switches are shown at 44 and after each tap changing operation has been completed and maintained closed until the succeeding operation is to be instituted. i

The system of Fig. 8 illustrates one manner in which" a plurality of vapor-arc devices may be I excited from but a single set of excitation transformers. winding of power transformer Ill is provided with taps l, 2, I, 4 and I intermediate each of which and theload circuit I2 is disposed a symmetrlcally conducting vapor-arc device of the type already described. The make-alive elements I I of each of the five vapor-arc devices A, B, C, D and E are excited from a transformer 22'- while elements IQ of the devices derive their excitation from a transformer 26. The primary winding 10 of transformer 28' is energized from the power source conductors ll through -a cir' cuit which includes a control switch while the primary winding I4 of transformer 22' is energized through a circuit which includes selector switches 15 to 19, inclusive, by the potential appearing across any one of the vapor are devices. The secondary windings BI and 82 of exciting transformer-22 supply, through suitable selector switches, the inner make-alive elements I. of the vapor arc devices, while the secondary windlugs 84 and II of transformer 26' similarly supply, through groups of selector switches, the outer make alive elements ill of the vapor arc devices A,to E, inclusive.

In operation of the tap-changing system of Fig. 8, the control or selector switches are manipulated in the following manner. Assuming that the load circuit I! is completely disconnected from transformer ID, the first step is to close switch 12 which places the equipment in condition for selective In the system shown, the secondary connection with any one of the taps l to 5, in-

elusive. To connect the load to tap I, 'close switches 41. This operation causes transformer 2! to excite the other make-alive elements I9 of vapor arc device A to thereby render it conductive.

To shift the connection from tap l to tap 2 close-switches 88 and IS. This sets up a circuit, influenced by the potential across device A; for

the inner make-alive elements of device B. Next open switches "to render devlceA non-conductive and to thereby effect the excitation of the inner make-alive elements of device B. Device B is now conductive. tion to the power source influenced circuits, close switches '04 which energizes the other make-alive elements of device B. Next, open switches it and I! to disconnect the inner make-alive elements "ofdeviceB. I In a similar. manner, a shift of power circuit connection from tap 2 totap I is effected by first closlng swltches I and 1 4, opening switches Q0, closing switches 12. and opening switches 9| and To shift from taps-to tap 4, close. switches and IL'openswltch II, close switches 88. and openswitches )4 and l1. s

V The remaining tsp change in thevoltage-ralslng direction, or from tap 4 to tap I, is effected by closing switches 84 'and I4, opening switches 95,.

switches l1 and opening switches 90 and closing 1.." Shifting of the tap connections in the opposite or voltage-lowering connection will be seen to be elfe'cted in'a manner which ls-culte comparable to that just explained. For example, toshift from tap I, in which the loadicnrrent lscarrled by.

C. to tap 4lln which the current To shift-its excltais carried by device D, switches 04 and I are first closed to set up a circuit for the inner-makealive elements I0of device D. Switches 01 are 0| and I0 are closed, switches 95 are opened,

switches 92 are closed and switches 0i and I8 are opened,

A shift from tap 3 to tap 2 is effected by closing switches 00 and 11, opening switches 02, closing switches 90 and opening switches 08 and 11.

Finally a shift from tap 2 back to top I iseffected by closing switches 98 and I6, openingswitches 00, closing switches 81 and opening switches 90 and I0. i

Disconnection of the load I2 from any one ofthe several taps may in the system of Fig. 8:be effected by opening the control switch I2. It will be apparent from the description oi the system just completed that the number of tapcontrolling vapor are devices which may, be" excited from a single pair of excitation transformers is not restricted to flve but may be any number desired.

In the event that it is desired to appLv vaporare devices to 'a tap changing system without utilizing the potential drop across one device to -eflect the excitation of the succeeding device in a tap changing operation, the combination shown in Fig. 9 may be employed. In Fig. 9 the load circuit I2 is energized from any one of a plurality of taps on the secondary winding of the power transformer I0" through circuits which include vapor are devices A to E, inclusive, intermediate which devices and the load is disposed a tap- I bridging auto-transformer I00. It will be recognized that were devices A to E to be replaced by a conventional type of contactor switches, the' system illustrated will be an exact reproduction of a well known type of single winding tap-changing scheme utilizing a tap-bridging auto-transformer. Hence, the vapor are devices, which are illustrated as being provided with but a single pair of make-alive elements I9, function merely to interrupt and establish the load circuits in the same manner as would engageabie contact type switches.

To supply excitation to the make-alive elements of the vapor-arc devices, a transformer I01, having four duplicate secondary windings I04 and a primary winding I05 energized from the power source circuit through a control switch I0'I, is provided. Intermediate the secondary windings of the transformer I02 and the make-alive elements'of the several vapor are devices is disposed a multi-pole control switch I08 in the manner illustrated. The several blades. of this switch are arranged for gang operation, the switch being capable of nine separate positions indicated by the pointer atthe right of the diagram;

To place the system in operation, control switch I" is first closed to energize exciting transformer I02. When switch I06 occupies position 1, as illustrated, onlythe make-alive elements of device A' will be'excited and the load circuit I 2 thus will be supplied through device A from tap, I. when switch I00 is moved to position 2, the makealive elements of the device B are also excited so that the load will then be supplied from the two adjacent transformer winding taps I and 3 through devices A and B which feed through opposite halves of auto-transformer winding I00. For such a'condition the load circuit will be influenmd by a voltage which corresponds to point 2 intermediate the taps I and l.

Advancement of the control switch to position 3 removes the excitation from device A and thus causes the load to be influenced by the voltage of tap 3. Advancement of the switch to position 4 effects the excitation of device B thereby causing the load to be influenced by the voltage corresponding to the point 4. Further advancement to position 6 removes the excitation from device B and raises the load circuit voltage to that of tap I. In a similar manner further advancement of the control switch progressively raises the load circuit voltage to that of points 6, 7, 8 and 9 of the secondary winding of transformer I0", in which last-named position all of the winding is connected to the load circuit I2 through the 1 ,8, 2 of the winding and lastly by that oftap I.

To eifect a complete disconnection of the load circuit from all of the. power transformer taps, regardless of the position of selector control switch I06, it is only necessary to open master control switch I01. Connection of the load circuit with the power transformer may be estab-, lished, as has been pointed out, by the mere closure of switch I01.

Although I have shown and described certain specific embodiments of my invention. I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the scope of the appended claims.

I claim as my invention: a

1. The .combinationwith an electrical winding having a plurality of taps, a circuit disposed for connection therewith, and a source of altematingcurrent voltage disposed to circulate current through said winding and circuit, oi' a vapor-arc device, which is capable of conduction when excited, disposed intermediate the circuit and each of the winding taps, means for selectively exciting each of said, devices from said alternat ing-current voltage, and means for additionally exciting each of said devices from a characteristic of any one of saidremaining devices when non-conductive.

2. The combination with an electrical winding having a plurality of taps, a circuit disposed for connection therewith, and a source of alternatingcurrent voltage disposed to circulate current through said winding and circuit, of a pair of vapor-arc devices each of which is capable of conduction when excited, means for disposing one oi. said devices intermediate the circuit and one of the winding taps and the other device intermediate the circuit and another of said taps, means for exciting each of said devices from said alternating-current voltage, and means for additionally exciting either of said devices from a characteristic of the other device when non-conductive. a

3. The combination, with a transformer wind- 6 ing having a plurality of taps, a circuit disposed for connection therewith, and a source of alternating-current voltage disposed to circulate current through said winding and circuit, of a vaporarc device, which is normally non-conductive and capable of full-wave conduction when excited, disposed intermediate the circuit and each of the winding taps, means for exciting from said alternating-current voltage any one of said devices to the winding tap associated with which it is desired that a circuit connection be established, and means for connecting for excitation by the voltage appearing across said first-named device when rendered non-conductive any other of said devices to the winding tap associated with which it is desired that the circuit connection be shifted upon a removal of excitation from said firstnamed device.

5. In a transformer tap-changing system disposed to shift the connection of an electrical circuit from one to another of the taps of a multitap transformer winding, said system comprising a vapor-arc device, normally non-conductive and -capable oi full-wave conduction when excited,

-tinuing the excitation of said first-named device to render it-non-conductive which allows its potential to build up sufflciently to eifect excitation of said second-named device, influencing the exciting means of" the second-named device by the transformer voltage, and disconnecting the exciting meansof this device from the first-named device.

6. In a transformer tap-changing system disposed to shift the connection of an electrical circuit from one to another of the,.taps of a multitap transformerv winding, said system comprising a vapor-arc device, normally non-conductive and capable of iull-waveconduction when ex-= cited, disposed intermediate the circuitand each of the winding taps and meansfor exciting each of said devices from the transformer voltage-and also [mm a characteristic of any one of the remaining devices when non-conductive, the method of effecting a change of circuit connection from a tap associated with a device maintained conductive by transformer-voltage excitation to any other tap without interruption of circuit current which comprises connecting the exciting means of the device associated with that other top to the first-named device, discontinuing the excitation of said first-named device to render it nonconductive which allows its said characteristic to effect excitation of said second-named device, influencing the exciting means of the secondhamed device by the transformer voltage, and disconnecting the exciting means of this device from the first-named device.

7. In combination with a source of alternatingcurrent power, a transformer having a winding provided with a plurality of taps, and a circuit disposed to be energized by said power source through a connection which includes one of the winding taps, means for shifting said connection from one to another of said taps without circuitcurrent interruption comprising two electronic devices, each of which is capable of full-wave conduction when excited, disposed intermediate the circuit and the winding, tap-selector switches disposed intermediate one of said-devices and the cuit from one'to another of the taps of a milititap transformer winding without circuit-current interruption, the combination of two vapor-arc devices, each of which is capable of full-wave conduction when excited, disposed intermediate the circuit and the winding, tap-selector switches disposed intermediate one of said devices, and the odd-numbered winding taps, tap-selector switches disposed intermediate the other of said devices and the even-numbered winding taps,

means for selectively exciting each of said electronic devices from the voltage of said alternating-currentsource, and means for additionally exciting either of said devices from a characteristic of the other device when non-conductive.

9. In combination with a transformer tap- I changing system adapted to operate in a loadcarrying circuit comprising a controllable ele'ctronic conduction device, a circuit breaker disposed to shunt said device for the purpose of eliminating the arc drop therethrough when the system is not being operated. s

10. In combination, ritransformer tap-changing system adapted to operate in a load-carrying circuit comprising a plurality of controllable va- -por-arc conduction devices through which the transformer circuit is selectively completed, and a circuit breaker pos d to shunt each of said devices for the purpose of eliminating-from the transformer circuit the arc drop of the device when no tap changing operation is being eifected.

11. In a. transformer tap-changing system adapted to operate in a load-carrying circuit, the combination of a plurality of vapor-arcdevices feach comprising two main electrodes of vaporiza,oso,ooa I adapted to operate in a load-carrying circuit, the combination of a plurality of pairs of asymmetrically conducting vapor-arc devices, each of said devices having a make-alive element, the devices in each pair being interconnected to be capable oi! symmetrical conduction, and means for selectively exciting the make-alive elements of the several pairs 01' devices.

13. In a transformer tap-changing system adapted to operate in a. load-carryins circuit, the combination of a plurality of controllable conducting means each comprising a pair of vaporarc devices each of which has an anode element,

a cathode element of vaporizable reconstructing material, and a make-aliveelement associated with each cathode, means for connecting the anode element of each device with the cathode element of the other device, and means for selectively exciting the make-alive elements of the several pairs of devices, each make-alive element of the'ex cited pair being acted upon during a1,- ternate halt cycles of transformer voltage in a manner to render the said pair of devices so in- 10 terconnected a lull-wave conductor.

JOHN W. DAWSON. 

